1. Field of the Invention
The present invention relates to an automatic focus adjusting (referred to as "AF" hereinafter) apparatus.
2. Related Background Art
A camera is generally provided with an automatic focus adjusting apparatus, and particularly, in recent years, a camera has necessarily been provided with an automatic focusing function.
Although there are many AF apparatus systems, video apparatus, such as video cameras, still video cameras and the like have recently remarkably been developed, a conventional AF system in which focus detected using the reflected light of an applied infrared has recently been changed to a system in which focus is detected and adjusted by determining an out-of-focus amount on the basis of the signal component which is extracted from the signal output from an image pickup element which varies depending upon a focusing state.
In such an AF system, a high-frequency component is extracted from the signal output from image pickup element, and a lens is controlled so that the high frequency level of the component is at a maximum.
In an AF apparatus, an area, i.e., a focus detecting area, from which a signal component, e.g., a high-frequency signal component, indicating a focusing state is extracted, is generally disposed on a predetermined portion near the central portion of an image plane. This focus detecting area is not provided over the whole image plane, but it is provided on a central portion of the image plane. Reasons for this are the following:
If the focus detecting area is excessively large, since the focus detecting area includes both a main subject and a background, focus cannot be detected with high precision. If the focus detecting area is excessively small, far and near competition between a main subject and a background occurs and thus makes a focusing state unstable.
Although typical recent AF systems have been described above, in these AF systems, a single focus detection requires a single vertical scanning referred to as "1 V" hereinafter) period necessary; for scanning the whole image plane because focus is detected using the signal output from an image pickup element.
However, about 12 to 13 times of scanning sometimes required for maintaining AF precision. Since a single AF scanning takes 1 V period, a about 200 ms is taken until AF scanning is completed, and a time lag at shutter release is thus increased. If an attempt is made to decrease the time lag, the number of times of AF scanning must be decreased, which produces an undesirable decrease AF precision.
There have been many prior applications concerning AF systems in which focus is adjusted on the basis of the signals output from an image pickup element or on the basis of signals from the focus detecting area provided on an image plane. Main examples of prior U.S. applications include the following:
(1) U.S. Ser. No. 17183 filed on Feb. 19, 1987, now abandoned PA1 (2) U.S. Pat. No. 4,994,920 PA1 (3) U.S. Pat. Ser. No. 240,915 filed on Sep. 6, 1988 PA1 (4) U.S. Ser. No. 154,078 filed on Feb. 9, 1988, now abandoned
However, there is no application concerning an AF system in which focus is adjusted by rapidly (within 1 V) reading only a portion of the image plane of an image pickup element. In any one of the AF systems, since a gate is applied to the image signals output at 1 V intervals, and only the signals within the focus detecting area are extracted, AF information can be obtained only on the 1 V unit, and an attempt cannot be made to increase the AF speed.